Interferometric Hetero-Detector Phase Measurement

We present a new method for the measurement of the phase of ultrashort optical pulses with high spectral resolution. This employs an interferometer with a monochromator in one arm. Two detectors operating in single photon and two-photon absorption regimes monitor the output from the interferometer. Comparison of the signals from the two detectors gives a measurement of the phase of the light pulse. The principles underpinning this technique are discussed and the experimental application to picosecond pulses is demonstrated.     

The effect of construction material, contents and room geometry on the sound field in dwellings at low frequencies

An experimentally validated finite element method is used to model the sound level in rooms at low frequencies. It is demonstrated that the dimensions of rectangular rooms strongly influence the sound pressure level difference. Additional factors were investigated which are not normally considered in the frequency range where diffuse sound field conditions can be assumed. Three effects were investigated: room damping due to wall vibrations, furniture, the effect of small deviations from simple rectangular shapes. It is confirmed by field measurements that the vibrations of masonry walls and floors introduce less damping than surfaces of lightweight construction. Assigning to the FE model a damping equivalent to a surface absorption of 0.02 reproduces the effect of walls of heavyweight construction. Damping equivalent to a surface absorption of 0.15 reproduces the effects of plastered timber-frame walls, floors and ceilings. The work was briefly extended to a room pair built with heavyweight and lightweight material of construction. The modification of the shape of the room frequency response highlights well the effect of material of construction. In-situ and laboratory measurements show that furniture has little effect on steady-state room response below 100 Hz. Modelling a wall recess smaller than 0.5 m improved the agreement between prediction and measurements but the assumption of a simple rectangular room remains appropriate.     

Modification of the surface state of rough substrates by two different varnishes and influence on the reflected light

Modification of the visual appearance when a rough surface is covered by a varnish is mostly attributed to the levelling of the substrate surface, which depends on the molecular weight of the varnish. The topography of varnished surfaces, however, has never been measured directly. Surfaces of varnishes applied over glass substrates of varying roughness were studied, therefore, using mechanical profilometry. Two different varnishes made with a low and a high molecular weight resin were studied. Both varnishes lower the r.m.s. roughness of the substrates and filter the high spatial frequencies. These results are amplified for the varnish containing the low molecular weight resin. The light reflected by the varnished samples is modelled from these topographical data. Its angular distribution, calculated from the probability density of slopes is presented, taking into account separately the air/varnish and the varnish/substrate interfaces. These analyses are presented in a back-scattering configuration. They show that varnishing significantly reduces the angular width of the reflected light and that this effect is magnified for the low molecular weight resin. Modelling furthermore shows that the influence of the roughness of the varnish/substrate interface is negligible in the total reflected light.     

腔内倍频调Q激光器中脉冲光场的时间不稳定性研究

以腔内倍频调Q激光器工作原理为基础,研究了多种形式抽运光分布的情况下倍频光的脉冲形状以及脉冲持续过程中光场分布的时间不稳定性,分析了倍频光光束质量随时间的变化规律.发现腔内倍频调Q激光器输出的倍频光光场分布在脉冲的持续过程中并不稳定,呈持续变化.对高斯型抽运的情况研究发现,减小腔长会使脉冲光场分布的不稳定性减小;而增加...     

不同控制参数下的弹簧摆

通过拉格朗日方程得出弹簧摆系统的动力学方程,并以频率比作为控制参数,利用Matlab软件对不同控制参数和初始摆角下的弹簧摆进行了数值模拟,从而直观地研究了弹簧摆的运动.     

用于氢原子1S-2S光谱研究的超窄线宽243nm半导体激光

近年来,随着激光稳频技术的发展,人们可以把972nm的外腔半导体激光器产生的激光进行放大和四倍频,并将它锁定到超稳定的光学法布里-珀罗腔上,从而可以获得Hz量级超窄线宽的243nm激光,使其成为研究氢原子1S-2S双光子跃迁的有力工具.文章主要介绍了超稳腔、四倍频243nm半导体激光器的研究进展及在氢原子1S-2S双光子跃迁精密光谱研究中的应用.     

铷原子饱和吸收光谱与偏振光谱对780nm半导体激光器稳频的比较

将激光频率锁定于合适的参考频率,可以有效地抑制激光器的频率起伏。本文采用铷原子D2线超精细跃迁线的饱和吸收光谱和偏振光谱分别获得鉴频曲线,通过电子伺服系统将频率校正信号负反馈到780 nm光栅外腔反馈半导体激光器外腔的压电陶瓷上的方法对激光器进行稳频。介绍了两种方法的基本原理和实验方案。与激光器自由运转300s时激光器典型的频率起伏约6.6 MHz相比,采用饱和吸收光谱和偏振光谱进行稳频,运转300 s时激光器典型的残余频率起伏分别约为1.5 MHz和0.6 MHz。分析表明,饱和吸收光谱稳频采用了相敏检波技术,需要对激光器进行频率调制,带来了额外的频率噪声,而偏振光谱稳频则是一种完全无频率调制的稳频方案。     

微波射频场在微波管中产生爆炸电子发射特性初步分析

对微波射频场在微波管内引起的场致发射和爆炸电子发射及等离子体的产生进行了分析,推导了等离子体产生强度与微波振幅、材料的电阻率、热传导系数、质量密度和比热容之间的关系,得到了晶须温度分布的表达式,通过数值解析的方式总结出在远大于微波周期的时间尺度上晶须温度提高随时间线性上升。在模型所述材料特性下,温度的上升率达到了3.22×1010 ℃/s,在100 ns量级就可以使晶须发生气化形成等离子体。     

氦氩射频容性放电发射光谱分析

光抽运亚稳态稀有气体激光器利用放电等离子体作为激光的增益介质.为掌握容性射频放电的放电参数对等离子体各项参数的影响的基本规律,利用等离子体发射光谱法研究了氦氩混合气体在不同装置、不同Ar组分、不同气压和不同射频注入功率下的等离子体参数.利用残留水蒸气产生的OH自由基A~2Σ~+→X~2Π的转动光谱分析获得气体温度;利用电子态光谱的玻尔兹曼做图法获得电子激发温度,利用Ar原子696.5 nm谱线的斯塔克展宽获得电子密度.结果表明:气体温度随气压增加略微上升,在一个大气压下改变组分和放电功率,气体温度变化不大;电子激发温度随总气压的下降而上升,且随着Ar组分的增加而略微下降;目前放电条件下的电子密度均在10~(15)cm~(-3)量级;长时间放电监测表明,残留的水蒸气会导致电子温度的下降,从而降低Ar亚稳态的产率.     

自积分式电容分压器的频率响应特性

为了测量电缆中传输的ns量级脉冲高电压,设计了自积分电容分压器并开展了频率响应特性分析。为分压器设计了不同的补偿电阻,并使用含有杂散参数的等效电路进行分析。仿真结果表明：分压器低频特性的主要影响因素是等效取样电阻与低压臂电容乘积得到的时间常数;高频特性主要受电容的杂散电感和取样电阻的杂散电容影响。增大时间常数扩展低频特性时,会导致杂散参数的影响加剧而使分压器高频特性变差。采用方波实验和扫频测量两种方法实测了不同参数分压器的频响特性。结果表明：补偿电阻为550 Ω的电容分压器频响上限超过2 GHz;但是低频特性不足,频率下限约为1.8 MHz;而补偿电阻为6.6 kΩ,且调整结构的电容分压器带宽为0.17~700 MHz,能够满足测试需求。